Typically, a printer prints in only one color, usually black. In certain applications, however, it is desirable to be able to print in a plurality of colors. For example, it may be desired to highlight particular information in the text being printed by displaying it in a different color, i.e. red. Also, since printers are often used to generate graphs, it is desirable to be able to readily distinguish different values or ranges of values in the graphs by using different colors for the different ranges.
Printing devices capable of printing in different colors have been available for some time. For example, the standard typewriter usually includes provision for typing in two different colors using an ink ribbon having upper and lower horizontal bands of different colors, e.g. red and black. A ribbon raising and lowering mechanism is used to raise the ribbon from its rest position selectively to either of two elevations depending upon whether the next character is to be printed in red or in black.
There also exist in the art high speed printers such as line printers and dot matrix printers capable of forming characters in a plurality of different colors. These prior printers also employ ink ribbons composed of upper and lower horizontal bands of different colors. In such prior printers, the ink ribbon is also shifted vertically relative to the print elements or print head between different vertical positions associated with the different color printing modes. Examples of such prior apparatus are disclosed in U.S. Pat. Nos. 3,583,315; 4,073,371 and 4,084,503. However, the apparatus disclosed there can only position the ink ribbon for selection between two different colors. Therefore, those printers would be unsuitable for producing texts and graphs requiring several different colors to differentiate the different categories of data in the text and graphs.
While it might appear to be obvious from the prior art that a wider selection of colors could be obtained simply by employing an ink ribbon composed of several different color bands and a printer mechanism for positioning the ribbon relative to the print head to permit selection of the different colors on the ribbon, it is not at all obvious how that can be accomplished in a reasonably simple and economical manner. That is, as evidenced from the above referenced art, the mechanisms used to position the ink ribbon to permit two color printing are already quite complex and expensive employing, as they do, numerous links, springs, toggles, pawls and electromechanical parts. It is not at all apparent, then, how any of those printers could be modified to enable it to print in even three different colors, let alone four or more colors.
With specific reference to the matrix printer disclosed in the above-mentioned U.S. Pat. No. 4,073,371, that apparatus has other distinct disadvantages, namely, the ribbon shifting mechanism produces ribbon travel problems and requires a cam which places an extra load on the carriage which inhibits its speedy movement from one printing position to the next. Also that mechanism requires a long radius arm to the ribbon shifting member and is quite bulky. Other prior apparatus rely on DC servos to achieve color selection and their response is quite slow.